1. Field of the Invention
The present invention relates to an optical recording medium, particularly to one aiming for improvement and the like of recording density of a write-once or a rewritable type optical recording medium also having a reproduction-only recording area.
2. Background of the Invention
An optical disk which is one kind of the optical recording medium generally has a spiral shaped track at a predetermined pitch p (for example, p=0.7 to 1.6 .mu.m ) in a signal recording area 2 on one surface (a signal surface) 4 of a substrate 3 made of optically transparent plastic as shown in FIG. 1A. The track consists of either continuous groove shaped concaves and convexes (groove 5 as a concave portion and land 6 as a convex portion) as shown in FIG. 1B or a pit row formed of continuous pits 8 as shown in FIG. 1C.
Of the optical disks, in an optical disk which makes it possible for a user to write in information (hereafter, referred to as a recordable optical disk) such as a write-once type and a rewritable type employing a phase change recording method or a magneto-optical recording method, what constitutes the main current is one which employs either the groove 5 or the land 6 in FIG. 1B as a recording area while employing the remaining one as a tracking light reflection area.
On the other hand, in the reproduction-only optical disk, what constitutes the main current is one in which the pit row where the pit 8 is continuous in FIG. 1C is employed simultaneously as a recording area and a tracking diffraction grating.
When information is to be recorded and reproduced, a laser light is irradiated from an optical pick-up (not shown) upon an opposite surface (read-out surface) 7 of the signal surface 4 while an optical disk 1 is rotated by the driving of a spindle motor (not shown).
Then, in a recordable optical disk, at a time of recording, information is written by its irradiating light in, for example, a record area on the land 6 as a mark which is equivalent to a pit in the reproduction-only disk, and at a time of reproducing, the written information is read out by a reflection light. Also, in order that the laser light for the recording and the reproduction is always irradiated on a predetermined track, tracking is carried out by detecting, for example, a reflecting light from the grooves 5 and the lands 6.
On the other hand, in the reproduction-only optical disk, read-out of the information and the tracking are respectively carried out by detecting a reflecting light and a diffraction light from the signal surface 4 on which the pit row 8 is formed.
Since the shape of such the track affects performance as a recording medium, it is demanded that the substrate 3 is most precisely manufactured. FIG. 2 illustrates a generally practiced manufacturing process of the substrate of an optical disk.
(1) Manufacture of a Master
Glass as a material for a master is worked to become a plate shape, its surface is ground sufficiently flat, washed and dried whereby a glass master 23 is manufactured.
(2) Painting of Photoresist
Photoresist, e.g., positive type resist which becomes soluble in alkali by exposure treatment 20 is painted about as thick as about 0.1 .mu.m on the glass master 23 and the photoresist 20 is dried by carrying out heat treatment on the glass master 23.
(3) Recording by a Laser Beam (Cutting)
The photoresist 20 is exposed to light by condensing a recording laser light 31 by an objective lens 32 and irradiating it on the photoresist 20 on the glass master 23. In a case of the recordable optical disk, the irradiation is carried out continuously and in a case of the reproduction-only optical disk, the irradiation is carried out intermittently. Concurrently, an exposure spot is fed in a radius direction of the master 23 at a constant feeding pitch, i.e., at an equal distance per one rotation while rotating the glass master 23 in a circumferential direction. As a result, in the case of the recordable optical disk, a latent image for a track consisting of a groove in a spiral shape at a constant interval is generated in the photoresist 20 whereas in the case of the reproduction-only optical disk, a latent image for a track consisting of spiral shaped pit rows at a constant interval is generated in the photoresist 20.
(4) Development
By developing the photoresist 20 in an alkaline developing solution, its exposure portion is removed. As a result, in the case of the recordable optical disk, a track pattern consisting of an alternation of a groove 25 and a land 26 in a spiral shape at a predetermined pitch is formed on the glass master 23. Also, in the reproduction-only optical disk, a track pattern consisting of continuous spiral shape pit rows 28 at a predetermined pitch is formed.
(5) Manufacture of a Stamper
By electroforming nickel on the glass master 23 and peeling off a formed nickel layer, a nickel master (stamper) 34 onto which the pattern on the glass master 23 is transferred is manufactured.
(6) Plastic Molding
By molding plastic as a material for the substrate of the optical disk through an injection molding method or the like using the stamper 34, an optical disk substrate 3 having the track consisting of the groove and the land or the pit row as shown in FIGS. 1A to 1C is manufactured. This substrate 3 is a replica of the glass master 23.
After the replica is manufactured, a recording film, a reflection film and the like (not shown) are formed on the signal surface 4 of the substrate 3 in the recordable optical disk, while in the reproduction-only optical disk, a reflecting film, a protection film and the like (not shown) are formed on the signal surface 4 of the substrate 3.
FIG. 3 shows a schema of whole the structure of an apparatus (a cutting machine) used for carrying out the cutting in the process in FIG. 2, and FIG. 4 shows detailed structure of its optical system. The cutting machine is formed of the following parts.
(1) Laser Apparatus 41 as a Light Source
As one example, a Kr ion laser apparatus with a wavelength of 413 nm is used.
(2) Recording Light Intensity Control Unit 42
An apparatus for eliminating instability of output from the light source to control the final recording light intensity for which a servo system employing an electro-optical crystal element (EO) 42a, an analyzer 42b, a photo-diode 42c, a recording light intensity control circuit 42d is used.
(3) Light Modulating Unit 43
This is an optical system provided with a light modulator 43 a on an optical path formed of beam splitters BS1, BS2, and convex lenses L1 and L2. The light modulator 43 a is used to form a pit of a length corresponding to a voltage level of an electric record signal, and to convert the voltage level of the record signal to a light intensity. For example, when the voltage level of the record signal consists of 2 values such as "0" and "1", a passing light is made on and off. As the light modulator is required to have performance capable of being used in a band of several tens of MHz, usually, an EOM (electro-optic crystal element modulator) and an AOM (acousto-optic crystal element modulator) are used.
(4) Beam Expander Unit 44
This is an optical system to expand a diameter of a beam of a recording laser light and a spot diameter of a condensed light is adjusted by its enlargement factor (magnification).
(5) Objective Lens 45
This is an optical system which condenses and irradiates the recording laser beam upon the photoresist 20 on the glass master 23.
(6) A Turntable 46 for Holding and Rotating the Glass Master 23 in a circumferential direction.
(7) Feeding Mechanism (not shown)
This is a mechanism for feeding an exposure spot of the record laser beam in a radius direction of the glass master 23 by holding the beam expander unit 44 and the objective lens 45 on a shifting stationary plate and shifting the shifting stationary plate by a motor and the like in a radius direction of the master 23.
(8) Servo System
This is for maintaining a distance between the master 23 and the objective lens 45 constant in a direction perpendicular to the surface of the glass master 23, and usually, a focusing laser 47 with a wavelength to which the photoresist is not photosensitive 20 is used.
With the use of a cutting machine having such a construction, the latent image of a spiral groove (or a pit row) at a constant interval is, as mentioned before, generated on the photoresist 20 by feeding the exposure spot of the record laser beam at a predetermined feeding pitch in the radius direction of the master 23 while rotating the glass master 23.
An fundamental construction of the optical system in the cutting machine is illustrated in FIG. 3 and FIG. 4, but in a case of the recordable optical disk, it is necessary at a stage of a manufacturing process of the substrate to beforehand record an address signal and the like which will become a marker when a user writes information. As a method for that, there are provided such a method by which exposure for a pit of the address signal is carried out at a position other than the optical system for the exposure spot for the groove, and another method by which a latent image for a serpentine groove is generated by vibrating (wobbling) the exposure spot for the groove in a radius direction of the master and an optical system corresponding to them is added to the cutting machine.
FIG. 5 shows a schema of construction of a cutting machine having a two-beam optical system corresponding to the wobbling. There is provided another optical system (referred to as channel Ch-B) other than the optical system (referred to as channel Ch-A) shown in FIG. 3 and FIG. 4, wherein the quantity of light given to the Ch-A almost equals the quantity of light given to the Ch-B by setting the transmittance of a beam splitter BS1 in a latter stage of the analyzer 42b to about 50%. However, the transmittance of the beam splitter--BS1 can be set to other than 50% corresponding to a necessary quantity of light for each of the channels.
The Ch-B has entirely the same construction as the light modulating unit 43 in the Ch-A as far as the light modulating unit is concerned, and by making an electric recording signal supplied to the light modulator 43a in the (Ch-A) and an electric recording signal supplied to the modulator 43a' in the Ch-B different, recording laser lights corresponding to different patterns (for example, the group in the Ch-A, the pit in the Ch-B) can be obtained.
On the optical path in the Ch-B, is provided a light deflector 51 such as an AOD (acousto-optical deflector) and the like, and depending on an electric signal inputted to the light deflector 51, the optical axis direction of the recording laser light in a direction of a light axis slightly oscillates within one plane. As a result, the exposure spot oscillates on the glass master 23. Further, according to a present format of an optical disk, as the wobbling is supposed to be carried out in a radius direction of the master, the light deflector 51 is so disposed that the exposure spot oscillates in a radius direction of the master 23 as shown in FIG. 6.
The optical axis of the recording laser light in the Ch-B is aligned with that of the recording laser light in the Ch-A by a polarizing beam splitter PBS through a light splitter BS3. Here, as a laser light is emitted from the laser apparatus 41 in a state of a linearly polarized beam, the recording laser lights at both channels before the polarized beam splitter PBS become the linearly polarized light of the same direction. As the polarized beam splitter let pass a linearly polarized light of a certain direction 100% but reflects the linearly polarized be am perpendicular to that direction 100%, in order for the recording laser lights at both the channels to reach the glass master 23 with maximum quantity of light, the linearly polarized light of one channel is sufficient to be rotated by 90 degrees.
Then, for example, in the Ch-A, half-wave plate 52 is provided at a front station of the polarized light splitter PBS, and the polarized light splitter PBS lets pass 100% the recording laser light in the Ch-A, wherein the direction of the linearly polarized light is rotated by 90 degrees by the half-wave plate 52, and makes the same incident on the light expander unit 44 while the linearly polarized light splitter PBS reflects the recording laser light in the Ch-B 100% and makes the same incident on the light expander unit 44.
Also, as the half-wave plate rotates direction of the linearly polarized light which is incident thereon at an incident angle of .theta. relative to a direction of a crystallographic axis within the plate surface by angle of 2.theta., if the incidence angle of the recording laser light in the Ch-A on half-wave plate 52 is adjusted to change a rotating angle in a direction of the linearly polarized light, because the transmittance of the recording laser light in the Ch-A in the polarized light splitter PBS varies from 0% to 100% a ratio between the quantities of light of the recording laser lights in both the channels can finally be adjusted.
The recording laser lights in both the channels, which have passed through the light expander unit 44 are condensed by the objective lens 45 and subjects the photoresist on the glass master 23 to the exposure. The exposure spots of both the channels are spaced apart by a minute distance in a radius direction of the glass master 23 (generally within one half a pitch of the track, that is, as much as about 1 .mu.m at the maximum). An adjustment to the end is carried out by giving the polarized light splitter PBS "a swing angle" so that a reflection angle of the recording laser light in the Ch-B at the polarized light splitter varies on the radius direction of the glass master 23 from a state in which the optical axes of the recording laser lights in both the channels completely coincide. As the exposure spot of the recording laser light in the Ch-B moves in the radius direction on the glass master 23 by the change in the reflection angle in the same manner as in the case of the wobbling shown in FIG. 6, the exposure spots in both the channels are spaced apart.
Next, a track format of the optical disk substrate will be described. In the past, "a single spiral structure" forming one-line spiral shaped recording track T as shown in FIG. 7A has been employed, but, recently various kinds of formats have newly been proposed in order to aim for high density of a recordable optical disk. One of them is a double spiral structure as shown in FIG. 7B to form 2-line spiral shaped recording tracks Ta and Tb.
As the double track structure has a fault in that jumping from either of the recording tracks Ta or Tb to another must be carried out in order to access all of recorded positions on the tracks, it is a format to be employed in case its merit in improving the record density far outweighs its fault.
There are following two kinds of method to form the double spiral structure.
As a first method, there is a method in which a feed pitch of the exposure spot at the time of cutting is set twice as many as the track pitch p to 2p to use both the groove and the land as an record area (called as a land/group record method). Generally, the width of each of the groove and the land is set to p each.
The first method effectively makes use of an area which has in the past been used only as a tracking guide groove out of the land and the groove to improve the record density and is thought to be the most reliable method for the high densification of a future optical disk.
As a second method, there is a method in which two spots away by the track pitch p in a radius direction are subjected to the exposure and its feed pitch is set two times the track pitch p to 2p at the time of the cutting (called "a 2 spot exposure method").
As a representative example for this method, there is "an intermittent wobbling method" which the applicant of the present invention proposes (U.S. patent application Ser. No. 08/823,879). This method is applied by modifying the form of the wobbling for recording the above-mentioned address signal and the like, in which, as shown in FIG. 8, assuming that one of the tracks Ta and Tb (Tb in the drawing) is made as a wobble track, its exposure spot (spot B) is vibrated while in the remaining track (Ta in the drawing), its exposure spot (spot A) is normally let to advance straightforward.
There is a problem with the normal wobbling in that, as the track pitch is made narrower, leakage (cross talk) record signals between adjoining tracks becomes larger, thereby making it difficult to read out the signal correctly. However, according to the intermittent wobbling method, because a wobbled track exists only at every other line, it is possible to sufficiently prevent the cross talk and correctly read out a signal even in a case the track pitch is narrow. This method is scheduled to be employed by the second generation MD-DATA, which is a kind of mini-disk.
The 2 spot exposure method is carried out, using by a cutting machine having a 2 light optical system as shown in FIG. 5.
By the way, the double spiral structure mentioned above is a track format proposed to aim for the high densification of the record area in the recordable optical disk and presupposes to have only a data record area consisting of the groove and the land.
In contrast to this, it is general for optical record media of various kinds including, for example, an optical disk to beforehand record inherent information concerning a format, and also, in addition to the beforehand recorded information of the various kinds, there exists an optical disk (is called a "partial ROM") in which a user can write information later, but no optical disk with a track format suitable for these has ever been proposed yet.